Microlithography (also called photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. In general, at each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as ultraviolet light. Next, the wafer with the photoresist on top is exposed to projection light through a mask in a projection exposure apparatus. The mask contains a circuit pattern to be projected onto the photoresist. After exposure the photoresist is developed to produce an image corresponding to the circuit pattern contained in the mask. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus typically includes an illumination system, a mask alignment stage for aligning the mask, a projection lens and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that may have the shape of a rectangular slit or a narrow ring segment, for example.
In current projection exposure apparatus a distinction can be made between two different types of apparatus. In one type each target portion on the wafer is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper. In the other type of apparatus, which is commonly referred to as a step-and-scan apparatus or simply scanner, each target portion is irradiated by progressively scanning the mask pattern under the projection light beam in a given reference direction while synchronously scanning the substrate parallel or anti-parallel to this direction. The ratio of the velocity of the wafer and the velocity of the mask is equal to the magnification β of the projection lens, for which usually |β|<1 holds, for example |β|=1/4 or |β|=1/100.
One aim in the development of projection exposure apparatus is to be able to lithographically define structures with smaller and smaller dimensions on the wafer. Small structures lead to a high integration density, which generally has a favorable effect on the performance of the microstructured components produced with the aid of such apparatus.
The minimum size of the structures that can be lithographically defined is approximately proportional to the wavelength of the projection light. Therefore the manufacturers of such apparatus strive to use projection light having shorter and shorter wavelengths. Among the shortest wavelengths currently used are 248 nm, 193 nm or 157 nm and thus lie in the deep (DUV) or vacuum (VUV) ultraviolet spectral range. The next generation of commercially available apparatus will use projection light having an even shorter wavelength of about 13.5 nm (EUV). However, the optical systems of such EUV apparatus are catoptric, i.e. they contain only reflective optical elements, but no lenses.
The correction of image errors (aberrations) is becoming increasingly important for projection objectives designed for operating wavelengths in the DUV and VUV spectral range. Different types of image errors usually involve different correction measures.
The correction of rotationally symmetric image errors can be comparatively straightforward. An image error is referred to as being rotationally symmetric if the wavefront deformation in the exit pupil is rotationally symmetric. The term wavefront deformation refers to the deviation of a wave from the ideal aberration-free wave. Rotationally symmetric image errors can be corrected, for example, at least partially by moving individual optical elements along the optical axis.
Correction of those image errors which are not rotationally symmetric is typically more difficult. Such image errors occur, for example, because lenses and other optical elements heat up in a rotationally asymmetric manner. One image error of this type is astigmatism, which may also be observed for the field point lying on the optical axis.
A major cause for rotationally asymmetric heating of optical elements is a rotationally asymmetric, in particular slit-shaped and/or off-axis, illumination of the mask, as is typically encountered in projection exposure apparatus of the scanner type. The slit-shaped illuminated field causes a non-uniform heating of those optical elements that are arranged in the vicinity of field planes.
A non-uniform heating may also occur with certain illumination settings. The term illumination setting refers to the angular distribution of the projection light bundles that impinge on points on the mask. The illumination setting is often described by the intensity distribution in a pupil surface of the illumination system. For example, with a dipole illumination setting only two poles arranged symmetrically with regard to the optical axis are illuminated in the pupil surface of the illumination system. A similar intensity distribution including two high intensity poles will also be observed in a subsequent pupil surface within the projection objective. This results in a rotationally asymmetric heating of lenses which are arranged in or in close proximity to a pupil surface.
The non-uniform heating results in deformations of the optical elements and, in the case of lenses and other elements of the refractive type, in changes of their index of refraction. If the materials of refractive optical elements are repeatedly exposed to the high energetic projection light, also permanent material changes may occur. For example, sometimes a compaction of the materials exposed to the projection light is observed, and this compaction results in local and permanent changes of the index of refraction.
The heat induced deformations and/or index changes alter the optical properties of the optical elements and thus cause image errors. Heat induced image errors often have a twofold symmetry. However, image errors with other symmetries, for example threefold or fivefold, or image errors characterized by completely asymmetric wavefront deformations also occur in projection objectives. Completely asymmetric image errors are often caused by material defects which are statistically distributed over the optical elements contained in the projection objective.
In order to correct rotationally asymmetric image errors, U.S. Pat. No. 6,388,823 B1 proposes a lens which can be selectively deformed with the aid of a plurality of actuators distributed along the circumference of the lens. The deformation of the lens is determined such that heat induced image errors are at least partially corrected.
WO 2007/017089 A1 discloses a similar correction device. In this device one surface of a deformable plate contacts an index matched liquid. If the plate is deformed, the deformation of the surface adjacent the liquid has virtually no optical effect. Thus this device makes it possible to obtain correcting contributions from the deformation not of two, but of only one optical surface. A partial compensation of the correction effect as it is observed if two surfaces are deformed simultaneously is thus prevented.
However, the deformation of optical elements with the help of actuators can also have some drawbacks. If the actuators are arranged at the circumference of a plate or a lens, it may be possible to produce only a restricted variety of deformations with the help of the actuators. This is due to the fact that both the number and also the arrangement of the actuators are fixed.
The aforementioned WO 2007/017089 A1 also proposes to apply transparent actuators directly on the optical surface of an optical element. However, it can be difficult to keep scattering losses produced by the transparent actuators low.
US 2009/0257032 A1 discloses a wavefront correction device which includes an optical element and a plurality of very thin electrical conductor stripes that are applied to a surface of the optical element or are integrated therein. In one embodiment arrays of conductor stripes are stacked one above the other, and a mechanism is proposed to restrict the heat dissipation to the crossing areas of the conductor stripes. This wavefront correction device makes it possible to produce a wide variety of temperature distributions within the optical element, and consequently a wide variety of rotationally asymmetric wavefront deformations can be corrected. However, light losses due to scattering can remain an issue.
An entirely different approach to deal with heat induced image errors is not to correct the errors, but to avoid that the errors occur altogether. This usually involves the locally selective heating or cooling of optical elements so that their temperature distribution becomes at least substantially symmetrical. Any residual heat induced image error of the rotationally symmetric type may then be corrected by more straightforward measures, for example by displacing optical elements along the optical axis.
The additional heating or cooling of optical elements may be accomplished by directing a heated or cooled gas towards the element, as it is known from U.S. Pat. No. 6,781,668 B2, for example. Similar cooling devices which direct cooled gas flows towards an optical element are also known from U.S. Pat. No. 5,995,263 and JP 10214782 A. In all these prior art devices all gas flows have the same temperature which can be adjusted with the help of a tempering device that cools the gas to a predetermined temperature. Control of the cooling effect appears to be exclusively achieved by changing the flow rate of the gas.
It has also been proposed to direct light beams onto selected portions of optical elements so as to achieve an at least substantially rotationally symmetric temperature distribution on or in the optical element. Usually the light beam is produced by a separate light source which emits radiation having a wavelength that is different from the wavelength of the projection light. This wavelength is determined such that the correction light does not contribute to the exposure of the photoresist, but is still at least partially absorbed by the optical elements or a layer applied thereon.
EP 823 662 A2 describes a correction system of this type. In one embodiment additional correction light is coupled into the illumination system of the projection exposure apparatus in or in close proximity to a pupil surface.
US 2005/0018269 A1 describes a correction device which makes it possible to heat up certain portions of selected optical elements using a light ray that scans over the portions to be heated up.
U.S. Pat. No. 6,504,597 B2 proposes a correction device which does not employ scanning light rays. Instead, correction light is coupled into selected optical elements via their peripheral surface, i.e. circumferentially.